CRYSTAL CHANNEL FILTERS 133 



over points of the two reactance curves are farther away from the band, 

 since this section is responsible for the outer pair of attenuation peaks. 

 The design of the filters consisted in determining values for the in- 

 ductance coils, condensers, and crystals, such that the reactance 

 curves of the lattice arms of the filter passed through infinity and 

 intersected with each other at the desired frequencies and, at the same 

 time, in determining the impedance level for all of the elements such 

 that the filters would have the right image impedances. 



The curves of Fig. 7 would seem to indicate a somewhat greater band 

 width for these filters than shown by the insertion loss characteristic 

 of Fig. 1. The reason for this can best be explained by referring to 

 the image impedance of one of the filter sections as shown in Fig. 8. 

 Within the band the image impedance is, of course, a pure resistance 

 which varies with frequency. It is about 800 ohms at mid-band 

 frequency and falls rapidly to zero near the edges of the band. Assum- 

 ing the effective resistance of the coils, which is about 100 ohms, as 

 belonging to the terminating impedances, the filter sections actually 

 work between impedances of about 700 ohms. This means that large 

 reflection losses occur at each end of each filter section near the edges 

 of the transmission band where the image impedance of the filter is 

 very small. It is these reflection losses that are responsible for the 

 actual transmission band being much narrower than it would be with 

 the filter sections terminated in their actual image impedances. The 

 filter sections are designed with 800 ohms image impedance at mid- 

 band frequency instead of 700 ohms to make the band flatter and 

 somewhat wider than it would be otherwise. 



When a number of band filters are operated in parallel it is generally 

 necessary to connect across the paralleled end a two-terminal network 

 to correct for the distortion that would otherwise be present in the 

 highest- and lowest-frequency filters in the group. A circuit of the 

 network used for this purpose with the channel filters is shown in Fig. 9. 



The filters employ crystal elements in order to obtain abrupt dis- 

 crimination between wanted and unwanted frequencies and at the 

 same time to secure low and uniform loss in their transmitting bands. 

 This characteristic must not only be obtained at the time the filters 

 are assembled and adjusted but must be maintained throughout the 

 service life of the filters and not appreciably affected by temperature 

 variations. This imposes severe stability requirements upon the 

 elements used in the filters. The crystal elements themselves are very 

 stable when properly designed and once adjusted will retain at a 

 given temperature their frequencies of resonance within one or two 

 cycles seemingly indefinitely. Their temperature coefficient is only 



